The existence of environmental toxins that damage specific neurons raises the possibility that certain human neurodegenerative disorders may be of environmental origin. The development and application of simple and non- invasive methods that can assess alterations in brain neurochemistry would be an important step in studying population groups exposed to environmental neurotoxins. The overall objective of this project is to validate and assess the usefulness of a simple non-invasive coincidence detector system (dual- probe) for the measurement of neuronal markers using positron emitting radioligands. These studies of pre-synaptic and post-synaptic neuronal markers will be carried out in normal persons and experimental animal models as well as persons exposed to environmental or occupational neurotoxins. In persons chronically exposed to environmental toxins, these neurochemical measurements may be able to detect early signs of central nervous system damage or dysfunction prior to behavioral or clinical manifestation of disease. This is important in view of the growing body of scientific evidence which indicates that perturbations in synaptic neurochemistry are related to behavioral dysfunction and neuropsychiatric disorders.